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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
EFFECT OF RAILWAY TRACK ELEMENTSPROPERTIES ON STRESSES DISTRIBUTION
Abdulhaq Hadi Abed Ali1, Atheer Naji Hameed1* and Sinan Khaleel Ibrahim1
The railway track elements (iron rail bars, concrete sleepers, crushed stone layers, base orballast layer and the subgrade layer) play an important role to resist the stresses resulting fromthe movement of trains on this track line, the basic purpose for the railway components totransfer resulting stresses safely to earth’s natural layer. In this paper we use the Track 3.1program developed by US army crops engineers to calculate the rail bending stresses and tiebending stresses as well as shear stresses or layers reaction of load hanging from the train andon the assumption that the material of installation layer (ballast) behavior as in elastic materials.Several attempts to study the effect of changing the thickness of the installation layer (ballast),the distance between the sleepers (tie spacing), sectional area of the installation panel andmodulus of elasticity calculated from the value of CBR test (California bearing ratio) as well asthe number of bolts( spike number) , on the rail bending stresses , tie bending stresses and thevertical stresses at the surface of installation layer (ballast) , and subgrade layer as well asreaction (shear) for the sleepers. From the results obtained we note that the rail bending stressesis less when increasing the thickness of the installation layer (ballast) ,when increase ballastgrade size ( aggregate gradation ) and increase the modulus of elasticity of the subgrade layer.While the tie bending stresses increases with increasing distance (tie spacing) between themand the number of screws which fasten it with the installation layer, and less with increase themodulus of elasticity of the subgrade layer and the sectional area of the panel installation.
Keywords: Railway track element, Track 3.1 program, Rail bending stress, Tie bending stress
*Corresponding Author: Atheer Naji Hameed,[email protected]
INTRODUCTIONThe railway track system plays an important role
in providing a good transportation system in a
country. A very large portion of the annual budget
1 Highway and Transportation Department, College of Engineering, Al-Mustansiriya University, Baghdad, Iraq, 2012-2013.
to sustain the railway track system goes into track
maintenance. In the past, most attention has been
given to the track superstructure consisting of the
rails, the fasteners and the sleepers, and less
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
attention has been given to the substructure
consisting of the ballast, the subballast and the
subgrade.
Even though the substructure components
have a major influence on the cost of track
maintenance, less attention has been given to
the substructure because the properties of the
substructure are more variable and difficult to
define than those of the superstructure (Selig and
Waters, 1994).
Ballast is the most important component of
the substructure because it is the only external
constraint applied to the track in order to restrain
it. Ballast is also important for providing the fastest
and most economical method of restoring track
geometry, especially at a subgrade failure
situation. However, ballast is also one of the main
sources of track geometry deterioration
(Tutumluer et al., 2006).
RAILWAY TRACK ELEMENTSTrack components are grouped into two main
components: The superstructure and
substructure as shown in Figure 1. The
superstructure refers to the top part of the track
that is the rails, the fastening system and the
sleepers, while the substructure refers to the
lower part of the track: that is the ballast, the
subballast and the subgrade (Selig and Waters,
1994).
Ballast
Ballast has many functions. The most important
functions are to retain track position, reduce the
sleeper bearing pressure for the underlying
materials, store fouling materials, provide
drainage for water falling onto the track, and
rearrange during maintenance to restore track
geometry. Thus, ballast materials are required to
Figure 1: Typical Standard Railway Track Element
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
be hard, durable, and angular, free from dust and
dirt, and have relatively large voids. Since ballast
is a type of granular material, behavior of such a
material is well documented in granular materials
literature (Fong, 2006).
Railroad ballast is the aggregate layer usually
installed between crosstie and subgrade. It
transfers the load impact from the tie and
distributes over the low strength subgrade soil.
Many factors may influence the ballast
behavior before and after tamping such as
aggregate gradation, shape, angularity, tie surface
texture, and ballast compaction level.
According to the American Railway
Engineering and Maintenance of Way Association
(AREMA), ballast aggregate should be open
graded with hard, angular shaped particles
providing sharp corners and cubical fragments
with a minimum of flat and elongated pieces
(maximum 5% by weight over 3 to 1 ratio)
(Tutumluer et al., 2006).
Several studies in the last decade have linked
coarse aggregate size and shape properties to
pavement performance (Aursudkij, 2007; Fair,
2003). In the pavement unbound aggregate base
courses, while compaction is important from a
shear resistance and strength point of view, the
shape, size and texture of coarse aggregates are
also important in providing stability [6]. Field tests
of conventional asphalt pavement sections with
two different base thicknesses and three different
base gradations showed that crushed-stone
bases gave excellent stability because of a
uniform, high degree of density and little or no
segregation (Rose et al., 2006). Rounded river
gravel with smooth surfaces was found to be
twice as susceptible to rutting compared to
crushed stones (Talbot, 1980).
Ballast is the crushed granular material placed
as the top layer of the substructure, in the cribs
between the sleepers, and in the shouldersbeyond the sleeper ends down to the bottom ofthe ballast layer. Traditionally, good ballastmaterials are angular, crushed, hard stones androcks, uniformly graded, free of dust and dirt, notprone to cementing action. However, due to the
lack of universal agreement on the specificationsfor ballast materials, availability and economicconsiderations have been the main factorsconsidered in the selection of ballast materials.Thus, a wide range of ballast materials can befound, such as granite, basalt, limestone, slag
and gravel. One of the main functions of ballastis to retain track position by resisting vertical,lateral and longitudinal forces applied to thesleepers. Ballast also provides resiliency andenergy absorption for the track, which in turnreduces the stresses in the underlying materials
to acceptable levels. Large voids are required inthe ballast for storage of fouling materials anddrainage of water falling onto the track. Ballastalso needs to have the ability to rearrange duringmaintenance level correction and alignment
operations (Tutumluer et al., 2006).
Sleeper or Ties
The main functions of sleepers are to distribute
the wheel loads transferred by the rails and
fastening system to the supporting ballast and
restrain rail movement by anchorage of the
superstructure in the ballast (Fong, 2006).
Subgrade
Subgrade is the foundation for the track structure.
It can be existing natural soil or placed soil. The
main function of the subgrade is to provide a
stable foundation for the track structure. Thus,
excessive settlement in the subgrade should be
avoided (Fong, 2006).
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
Stresses on Railway Track
There are two main forces which act on ballast.
These are the vertical force of the moving train
and the “squeezing” force of maintenance
tamping. The vertical force is a combination of a
static load and a dynamic component
superimposed on the static load. The static load
is the dead weight of the train and superstructure,
while the dynamic component, which is known
as the dynamic increment, depends on the train
speed and the track condition. The high squeezing
force of maintenance tamping has been found to
cause significant damage to ballast. Besides
these two main forces, ballast is also subjected
to lateral and longitudinal forces which are much
harder to predict than vertical forces.
The dead wheel load can be taken as the
vehicle weight divided by the number of wheels.
The static load from the dead weight of the train
often ranges from about 53 kN for light railpassenger services to as high as 174 kN for heavyhaul trains in North America. The dynamicincrement varies with train section as it dependson track condition, such as rail defects and trackirregularity.
The static wheel load distribution was obtainedby dividing known individual gross car weights bythe corresponding number of wheels, and thedynamic wheel load distribution was measuredby strain gauges attached to the rail.
The vertical wheel force is distributed througha number of sleepers. The number of sleepersinvolved is highly dependent on the sleeperspacing and the rail moment of inertia.
The vertical downwards force at the rail-wheelcontact points tends to lift up the rail and sleeper
some distance away from the contact point, as
shown in Figure 2.
Figure 2: Uplift of Rails
Source: Selig and Waters (1994)
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
The uplift force depends on the wheel loads
and self-weight of the superstructure. As the wheel
advances, the lifted sleeper is forced downwards
causing an impact load, which increases with
increasing train speed. This movement causes
a pumping action in the ballast, which increases
the ballast settlement by exerting a higher force
on the ballast and causing “pumping up” of fouling
materials from the underlying materials in the
presence of water.
It is also noted that the impact load increases
with the increase in track irregularity or differential
settlement (i.e., impact load increases with the
increase in the size of the gap underneath the
sleeper). The increase of impact load would then
lead to an increase in ballast settlement and lead
to a larger gap underneath the sleeper. Thus,
track geometry tends to degrade in an
accelerating manner.
The lateral force is the force that acts parallel
to the long axis of the sleepers. The principal
sources of this type of force are lateral wheel force
and buckling reaction force. The lateral wheel
force arises from the train reaction to geometry
deviations in self-excited hunting motions which
result from bogie instability at high speeds, and
centrifugal forces in curved tracks. These types
of forces are very complex and much harder to
predict than vertical forces. The buckling reaction
force arises from buckling of rails due to the high
longitudinal rail compressive stress which results
from rail temperature increase. The longitudinal
force is the force that acts parallel to the rails.
The sources of this force are locomotive traction
force including force required to accelerate the
train, braking force from the locomotive cars,
thermal expansion and contraction of rails, and
rail wave action.
The vertical wheel force is distributed through
a number of sleepers. The number of sleepers
involved is highly dependent on the sleeper
spacing and the rail moment of inertia. Conducted
a parametric study using the GEOTRACK
computer program, which is a three-dimensional,
multi-layer model for determining the elastic
response of the track structure. They found that
as the sleeper spacing increased from 250 mm
to 910 mm, the load applied to the sleeper
beneath the wheel increased by a factor of about
4. They also found that for an increase of rail
moment of inertia from 1610 cm4 to 6240 cm4,
the load applied to the sleeper beneath the wheel
decreased by 40% (Selig and Waters, 1994).
TYPES OF STRESSES EFFECTON RAILWAY1. Bending stress
2. Ballast surface stress
3. Tie bending stress
4. Tie reaction
5. Subgrage stress
TRACK 3.1 PROGRAMSThe program which is used to calculate railway
track element is called “Track 3.1” developed by
US army Corps of engineers, transportation
system center, more details are shown on the
Plate 1 and input variables shown in Table 1.
SENSITIVITY ANALYSISSensitivity analysis has been made to illustrate
the effect of various parameters on pavement
structural design. Due to the complex interactions
among the large number of parameters, it is
difficult to present a concise but accurate picture
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
on the effect of a given parameter. This is because
the effect of such variable depends not only on
the parameter itself but also on other parameters.
Conclusions based on a set of parameters may
not be validated if some of the other parameters
are changed. The best approach is to fix all other
parameters at their most reasonable values while
varying the parameter in question to show its
effect as shown in the following Table.
DATA ANALYSIS
Effect of Ballast Thickness, it can benoticed from Figure 3
Rail Section
1) The bending stresses at rail decreased with
increased ballast thickness.
2) The tie reaction force increased with increased
ballast thickness.
3) The vertical stress at surface of the ballast
layer increased with increased ballast
thickness.
4) The vertical stress at the top of the subgrade
layer decreased with increased ballast
thickness.
Plate 1: Track 3.1 Program Menu
3 18 7"*9"=257 1 ¾ 1 1750 0
6 20 7"*8"=229 2 ½ 5 8750 1
9 22 6"*8"=144 3 ¼ 10 17500 2
12 24 4 loose earth 20 35000 3
15 30 52500 4
20 40 70000
36 50 87500
Table 1: Input Variables
Effectof Ballast
Depth(in)
Effectof Tie
Spacing(in)
Effect of TieCross-Section
(I, in4)
Effect ofBallast size (in)
Effect of SubgradeModulus (psi)
CBREs =
1750*CBR
Effect ofSpike No.
Ra
ilw
ay
Tra
ck
Ele
me
nt
Va
ria
ble
s (
No
. o
f T
rial
s)
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
Figure 3: Effect of Ballast Thickness
5) The tie bending stresses not significant effect
by ballast thickness.
Joint Section
Generally; the response behavior is the same in
rail section but less than it.
Effect of Tie Spacing , it can be seen fromFigure 4
Rail Section
1) The bending stresses at rail not significant
effect with tie spacing.
2) The tie reaction force increased with increased
tie spacing.
3) The vertical stress at surface of the ballast
layer increased with increased tie spacing.
4) The vertical stress at the top of the subgrade
layer increased with increased tie spacing.
5) The tie bending stresses increased with
increased tie spacing.
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
Joint section
Generally; the response behavior is the same in
rail section but less than it.
Effect of Changing Tie Cross Section , itcan be noticed from Figure 5
Rail Section
1) The bending stresses at rail not significant
effect with changing tie cross section.
2) The tie reaction force not significant effect with
changing tie cross section.
3) The vertical stress at surface of the ballast
layer decreased with increased tie cross
section.
4) The vertical stress at the top of the subgrade
layer decreased with increased tie cross
section.
Figure 4: Effect of Tie Spacing
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
5) The tie bending stresses decreased with
increased tie cross section.
Joint Section
Generally; the response behavior is the same in
rail section but less than it.
Effect of Changing Ballast Grade Size, itcan be seen from Figure 6
Rail Section
1) The bending stresses at rail reduced with
Figure 5: Effect of Tie Cross-Section
increasing ballast grade size due to increased
aggregate stiffness.
2) The tie reaction force increased with increased
ballast grade size due to increased ability of
ballast layer to resist.
3) The vertical stress at surface of the ballast
layer decreased with reduced ballast grade
size due weak in ballast layer.
4) The vertical stress at the top of the subgrade
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
layer not significant effect with changing in the
ballast grade size.
5) The tie bending stresses not significant effect
with changing in the ballast grade size.
Joint Section
Generally; the response behavior is the same in
rail section but less than it.
Effect of Strength of Subgrade it can benoteced from Figure 7
Rail section
1) The bending stresses at rail reduced with
increasing strength of subgrade (increasing
CBR value) due to increased subgrade
stiffness.
Figure 6: Effect of Ballast Grade Size
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
2) The tie reaction force increased with
increased subgrade strength due to increased
ability of subgrade layer to resist.
3) The vertical stress at surface of the ballast
layer increased with increased subgrade
strength due strong in subgrade layer.
4) The vertical stress at the top of the subgrade
Figure 7: Effect of Subgrade Strength
layer increased with increased subgrade
strength.
5) The tie bending stresses decreased with
increased subgrade strength.
Joint Section
Generally; the response behavior is the same in
rail section but less than it.
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
Effect of Spike Number, it can be seen fromFigure 8
Rail Section
1) The bending stresses at rail not significant
effect with increasing spike number.
2) The tie reaction force not significant effect with
increasing spike number.
3) The vertical stress at surface of the ballast
layer increased with increased spike numberdue to stable of tie plate.
4) The vertical stress at the top of the subgradelayer increased with increased spike number.
5) The tie bending stresses increased withincreased spike number.
Joint Section
Generally; the response behavior is the same inrail section but less than it.
Figure 8: Effect of Spike Number
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Int. J. Struct. & Civil Engg. Res. 2013 Atheer Naji Hameed et al., 2013
CONCLUSIONFrom the data analysis can be conclude:
1) The bending stresses at rail increased withdecreased ballast thickness and reduced withincreased ballast grade size and so reducedwith increased subgrade strength (CBRvalue).
2) The tie reaction force increased withincreased ballast thickness, tie spacing,ballast grade size and subgrade strength.
3) The vertical stress at surface of the ballastlayer increased with increased ballastthickness, tie spacing, subgrade strength andspike numbers but reduced with increased tiecross section and ballast grade size.
4) The vertical stress at the top of the subgradelayer decreased with increased ballastthickness, tie cross section but increased withincreased subgrade strength, spike numberand tie spacing.
5) The tie bending stresses increased withincreased tie spacing, spike number butdecreased with increased tie cross sectionand subgrade strength.
6) The response parameters in rail section aremore than in joint section.
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